This application relates to a new form of zeolite beta and to its use as a catalyst in the alkylation of aromatics. More particularly, this application relates to a zeolite beta which shows substantially greater stability and catalyst life when used in the alkylation and transalkylation of aromatics. It is contemplated that the catalysts of this invention may be particularly valuable in cumene production via alkylation of benzene with propylene. For ease and simplicity of exposition, the following description will make specific reference to the use of our catalyst in the alkylation of benzene with propylene to afford cumene, but it is to be recognized that this is done solely for the purpose of clarity and simplicity. We shall make frequent reference to the broader scope of this application for emphasis.
Cumene is a major article of commerce, with one of its principal uses being a source of phenol and acetone via its air oxidation and a subsequent acid-catalyzed decomposition of the intermediate hydroperoxide, ##STR1## Because of the importance of both phenol and acetone as commodity chemicals, there has been much emphasis on the preparation of cumene and the literature is replete with processes for its manufacture. Certainly the most common and perhaps the most direct method of preparing cumene is the alkylation of benzene with propylene, especially using an acid catalyst. See "Encyclopedia of Chemical Processing and Design," J. J. McKetta and W. A. Cunningham, Editors, V. 14, pp 33-51 (1982). Even though a high conversion of propylene and a high selectivity to monoalkylated products are two major prerequisites of any commercially feasible process, other constraints must be satisfied.
The predominant orientation of the reaction of benzene with propylene corresponds to Markownikoff addition resulting in cumene. However, a small but very significant amount of the reaction occurs via anti-Markownikoff addition to afford n-propylbenzene (NPB). The significance of NPB formation is that it interferes with the oxidation of cumene to phenol and acetone, and consequently cumene used for oxidation must be quite pure with respect to NPB content. Since cumene and NPB are difficult to separate by conventional means, as for example distillation, a constraint in the production of cumene by the alkylation of benzene is that the n-propylbenzene formed be minimized relative to cumene. An observation pertinent to this facet of alkylation is that the relative amount of NPB formation increases with increasing temperature. Thus, from the standpoint of minimizing NPB formation it is desirable to perform the alkylation at as low a temperature as possible. Stated differently, minimizing NPB requires avoiding high reaction temperatures.
Turning to the catalysts used in aromatic alkylation, solid acid catalysts are quite desirable from the viewpoint of designing a continuous process. It is unnecessary to articulate here a litany of solid acid catalysts used in aromatic alkylation; suffice it to say that many are described and among these zeolitic catalysts have received special attention. Whatever catalyst is used, deactivation is a feature which can not be avoided but is to be minimized to the extent possible. For zeolitic catalysts deactivation usually results from the accumulation of polyalkylated products on the catalyst surface and within the zeolite channels, and it has been observed that the rate of deactivation decreases with increasing reaction temperature. Thus, minimizing catalyst deactivation generally suggests performing the alkylation at relatively high reaction temperatures. Thus it is clear that attempts to decrease catalyst deactivation by effecting reaction at high temperatures is at variance with attempts to minimize NPB formation by effecting reaction at low temperatures.
What is required in an optimum catalyst for, e.g., cumene production, is a catalyst with sufficient activity to effect alkylation at acceptable reaction rates at temperatures sufficiently low to avoid unacceptable NPB formation while exhibiting the slow catalyst deactivation usually associated with higher reaction temperatures. Because zeolite beta shows substantially greater activity than other zeolites, it has received close scrutiny as a catalyst in aromatic alkylation; see, e.g., Innes et al., U.S. Pat. No. 4,891,458, Shamshoum et al., U.S. Pat. No. 5,030,786, and EP 432,814 inter alia. However, it is found that zeolite betas as described still deactivate at unacceptably high rates at the low temperatures desired to minimize NPB formation. In order for a commercial process based on zeolite beta to become a reality it is first necessary to either increase catalyst activity--i.e., increase the rate of cumene production at a given temperature--or to decrease catalyst deactivation--i.e., increase catalyst lifetime so as to increase cumene production prior to catalyst regeneration. This application focuses on making modifications to native zeolite beta to afford a catalyst showing decreased deactivation relative to other zeolite betas.
Without wishing to be bound by any one particular theory, the rationale employed in our approach assumed catalyst deactivation resulted from polyalkylation of aromatics, perhaps with a minor contribution from oligomerization, especially where the propylene concentration is quite large. We further assumed that polyalkylates (and other deactivating materials) formed mainly as a consequence of strong acid sites on the zeolite surface. We then concluded that formation of deactivating materials could be reduced by removing the stronger acid sites on the zeolite surface, especially by converting the strong acid sites to weaker ones ineffective, or less effective, in producing deactivating materials. We have found that treating templated zeolite beta with a low concentration of a strong mineral acid followed by calcination affords a superior zeolite beta. The order of treatment is critical; acid washing a calcined zeolite beta is largely ineffective! We believe that our treatment affects only the nature of surface acid sites, as shown by an unchanged silicon:aluminum surface ratio, and a changed surface aluminum 2p binding energy as determined by x-ray photoelectron spectroscopy.